There have been developed inkjet inks for industrial use which can be printed directly on a non-absorptive medium such as a polyvinylchloride sheet. Examples of these inkjet inks are cited as: a solvent ink in which an organic solvent is used as a vehicle of an ink; and a UV ink containing a polymerizable monomer as a primary component of an ink. A solvent ink is dried by evaporating its solvent to the air, as a result, a solvent ink has a problem of emitting a large amount of VOC, which becomes a social problem in recent years. There are other concerns for a worker about an odor or an effect for the safety. Therefore, it is required to provide equipment for making sufficient ventilation. A UV ink is made harden immediately after printing, therefore, an emission of VOC is close to zero, but many monomers to be used in the ink may have problem of skin sensitization. Further, there are requirement of incorporating an expensive UV light source into a printer, and it cannot be used for printers for every field. Moreover, when printed on a glossy type sheet, these inks cause extremely lusterless at the ink deposited portion, resulting in being difficult to have an image having excellent image quality.
In the above-described background, there has been developed an ink which can be printed also directly to a non-water absorptive recording medium, by using a water-based ink containing water as a major component which has been widely used in homes and has reduced effects on the environment.
There has been proposed a water-based ink containing a water miscible solvent selected from glycols and glycol ethers in Patent Document 1. Moreover, there has been proposed an ink containing a graft co-polymer binder which contains a hydrophobic backbone and a non-ionic and hydrophilic side-chain, wherein the graft co-polymer binder is soluble in a water-based vehicle and is not soluble in water. However, the inventors of the present invention studied the method disclosed in Patent Document 1 and found that the method disclosed in Patent Document 1 resulted in insufficient image qualities such as an ink mixing caused by coalescence of ink droplets deposited adjacently on a non-absorptive recording medium and a patchy pattern caused by gathering droplets at solid image having uniform medium density or color bleeding in multi-color printing (phenomenon in which ink droplets are mixed and a smear is produced in the boundary area of the image having a different color). Further, durability of the obtained image was also insufficient. Further, ejection failure was found when continuously using inkjet head in the method of Patent Document 1. Generally, ejection failure can be recovered by maintenance with some frequency. However, recovery through maintenance in the method of Patent Document 1 did not achieve satisfactory level.
Patent Document 2 discloses that styrene-acrylic acid copolymer is added to an ink. Copolymerizable polymer containing styrene enables to have high glossiness. However, on the other hand, it often tends to reduce abrasion resistance and adhesiveness in case of an image directly printed on a hydrophobic substrate.
Copolymer between styrene and α-methyl styrene and acidic monomer such as acrylic acid is commercially available. When employing this copolymer to an inkjet ink, it enables to have high glossiness, however, it exhibits extremely bad abrasion resistance and adhesiveness. Even though reasons are not understood in detail, it is supposed that since styrene has poor adhesiveness to substrate or the film is too hard and lack of flexibility, whereby crack or peel causes in the film because of inability to follow to distortion of soft vinyl chloride.
On the other hand, all acryl type polymers without including styrene are also commercially available, in which methyl methacrylate is used instead of styrene. However, this type without including styrene has issue which tends to decrease glossiness of an image.
Patent Document 3 discloses that water-soluble acryl resin without containing styrene is added to an ink. By using a resin coated pigment as pigment dispersion, this ink enables to have a certain extent of image durability (abrasion resistance), even when directly printed on vinyl chloride substrate. However, in the market, durability against further friction is required and this ink does not enable to have enough abrasion resistance for further friction as it is now.
Further, when these inks were stored for a long period of time, it was seen a phenomenon in some inks that the surface tension of ink rose rather than before storage. It was also found issues that ink was repelled or a dot contracts resulting in causing a hollow defect in an image, when such ink was printed on a hydrophobic substrate after prolonged storage.